It is important to ensure that concrete is frost durable in environments where it might be subjected to moisture and subsequent freezing and thawing cycles. This is done by adding specialized surfactants that stabilize air while mixing concrete. The air voids allow space for water to move during freezing. The size distribution of the voids is very important to the frost durability of concrete. Currently, there are no testing methods that allow the spacing of these voids to be determined in the fresh concrete in less than 30 minutes. Instead, either a volume of air is specified or testing must be postponed until the concrete has hardened. With hardened concrete, it can be cut and then polished and the voids on the surface can be measured and counted with American Society for Testing and Materials (“ASTM”) procedure C 457. This method aims to measure the air content, spacing factor, and specific surface of the concrete. This is a very time consuming and costly process.
Currently one test method exists to determine the size distribution of an air void system in fresh concrete. This proprietary test is called the Air Void Analyzer (“AVA”). This test involves injecting mortar collected from concrete into a thinning liquid that is being stirred. The air voids in the mortar are liberated and they slowly rise to the top of a column of water. A petri dish is attached to a scale or a strain gage at the top of the apparatus to measure the buoyancy force created by the air voids over time. From this data, various qualities such as the air content, spacing factor, and specific surface can be estimated.
The accuracy of this test has been called into question. It also takes more than 30 minutes to complete and the specialized testing device is very costly and cannot readily be used on a jobsite. The results are highly variable due to any number of factors, including: vibration, operator, sampling process, and time duration between sampling and testing.
Since the size of bubbles cannot currently be reliably measured in fresh concrete, it is common to use a test that instead measures the volume of air in concrete. There are three common tests that are able to do this. The most commonly used test is the ASTM C 231 pressure meter. This test is based on the observation that the only material that is readily compressible inside of confined concrete that has not hardened is the air voids. Since these pressures are low, the response is assumed to be linear and Boyle's Law can be used to estimate the volume of the air in the concrete. In brief, the associated method of testing involved connecting two chambers together. One chamber was initially left empty and the other was filled with the subject concrete of a known volume. Water was then added above the concrete to ensure the bottom chamber is filled. The top chamber of a known volume was then increased in pressure to about 14.5 psi. The top chamber was then opened into the bottom chamber to provide fluid communication and combined chambers were allowed to reach an equilibrium pressure. From this pressure, the total volume of air within the concrete can be estimated. As stated previously, this is the most widely used test method to determine the air content of the mixture. One problem with this approach is that it can only measure the volume of air and does not give information about the void size distribution.
Another commonly utilized test is the ASTM C 457 Hardened Air Void Analysis, which can be used to examine the air void system in hardened concrete. However, one problem with this sort of analysis is that results can take weeks to obtain. This makes it impossible to evaluate the acceptability of the freeze thaw durability before the concrete has been poured and hardened. The slow process that is hardened air void analysis is a cause for sluggish laboratory studies in which large data sets can be difficult and tedious to produce.
“Spacing factor” is a measure of half the average spacing between the average sized air void in the paste and is determined through investigation of the hardened concrete as described in ASTM C 457. Freeze thaw testing has shown that concrete with a spacing factor less than 0.008 in. is sufficient to resist frost damage and the American Concrete Institute 201 uses this value as one of its standards. Research has shown that different combinations of admixtures can have a significant impact on the size of the bubbles stabilized in concrete. Because of this, different volumes of air are required to achieve this critical spacing factor. However, it should be noted that these required air volumes can change with the temperature, mixing method, placement technique, and combination of other admixtures used. For example, if an air-entraining agent is used then about 3.5% air is required before satisfactory frost performance is achieved. If a polycarboxylate (PC) water reducer or dispersant is used with an AEA then a 7.5% air content may be necessary to achieve a spacing factor of 0.008 in.
Heretofore, as is well known in the concrete testing arts, there has been a need for a system and method of determining the air void size distribution in fresh concrete that is more cost effective and timelier than current methods. Accordingly, it should now be recognized, there exists, and has existed for some time, a very real need for a method of fresh concrete evaluation that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.